1. Field of the Invention
The present invention relates to a demodulator included in, e.g., a digital radio communication system for compensating for nonlinear distortion ascribable to an amplifier and a radio communication system using the same.
2. Description of the Background Art
It is a common practice with a demodulator included in a digital ratio communication system to compensate for nonlinear distortion. Japanese Patent Laid-Open Publication No. 11-136302, for example, pertains to a distortion compensating circuit of the type using an envelope detection type of linearizer. This type of linearizer compares the amplitude and phase of a digital signal to be sent and those of a signal fed back to thereby estimate the distortion of an amplifier, and then executes predistortion processing with the signal to be sent for thereby compensating for distortion. The circuit taught in the above document detects and compensates for the deviation of the gain of an orthogonal modulator and modulates with the orthogonal modulator four points of data inclined by 45xc2x0 relative to an I and a Q axis in baseband. The circuit then reads the resulting output of the orthogonal modulator with a linear detector and then performs calculation with the read output for thereby compensating for the error of the orthogonal modulator in the degree of orthogonality.
Japanese Patent Laid-Open Publication No. 8-163198 discloses a nonlinear distortion compensating device for compensating for distortion to which a signal modulated by a transmitter is subjected before it is demodulated by a receiver. The compensating device determines mean signal points with a mean signal point position calculating circuit and then identifies a signal point corresponding to a mean signal point position that is closest to an input signal point.
Further, Japanese Patent Laid-Open Publication No. 4-291829 proposes a system for compensating for nonlinear distortion ascribable to an amplifier even when the phase of a modulated wave varies. For this purpose, the system obtains the shift of the modulation phase of a modulator beforehand with respect to a factor that causes the phase of an amplified modulated wave to vary. The system then compensates for the shift of the modulation phase in accordance with the variation of the above factor.
However, the conventional compensation schemes described above cannot sufficiently compensate for nonlinear distortion, as will be described specifically later.
Technologies relating to the present invention are also disclosed in, e.g., Japanese. Patent Laid-Open Publication No. 6-152676.
It is an object of the present invention to provide a demodulator capable of compensating for nonlinear distortion with simple circuitry without making design sophisticated and a radio communication system using the same.
It is another object of the present invention to provide a demodulator capable of making, even when nonlinear distortion characteristics vary due to the modification of an amplifier included in, e.g., a communication apparatus, redesigning of a modulator or a demodulator needless and surely executing distortion compensation, and a radio communication system using the same.
A demodulator for a radio communication system of the present invention includes a demodulator for demodulating an input modulated wave to thereby output a demodulated signal. A signal point position detector detects the signal point position of the demodulated signal on orthogonal coordinates. An error detector detects the shift of the signal point position detected and the original signal point position of the demodulated signal. An outputting circuit outputs first information representative of the shift and second information representative of the signal point position. A generating circuit generates compensation information in accordance with the first information and second information. An adding circuit adds the compensation information to the demodulated signal to thereby output a sum signal. The sum signal is output from the demodulator as a distortion-compensated signal.